Air-based air conditioning systems typically comprise an air conditioning unit, which is arranged, for example, in a wing root of the aircraft, and which is supplied with compressed process air that is generated by a compressor or bled off from an engine or an auxiliary power unit (APU) of the aircraft. During flight operation of the aircraft, usually engine bleed air is used so as to supply the air conditioning unit of the aircraft air conditioning system with compressed process air. During ground operation of the aircraft the air conditioning unit of the aircraft air conditioning system, however, typically is supplied with compressed process air from the auxiliary power unit of the aircraft. In the air conditioning unit, the process air, upon flowing through at least one heat exchanger as well as through various compression and expansion units, is cooled and expanded.
The heat exchanger(s) of the air conditioning unit is/are cooled by ambient air provided to the heat exchanger(s) via a ram air channel. During flight operation of the aircraft, ambient air is conveyed through the ram air channel due to a pressure difference building up between a ram air channel inlet and a ram air channel outlet. During ground operation of the aircraft, a conveying device such as, for example, a fan is operated in order to provide for a sufficient mass flow of ambient air through the ram air channel. Cooled process air exiting the air conditioning unit finally is supplied to a mixing chamber where it is mixed with recirculation air recirculated from an aircraft region to be air conditioned. The mixed air from the mixing chamber, via respective mixed air lines, is supplied to the aircraft region to be air conditioned such as, for example a passenger cabin which may be divided into a plurality of air conditioning zones or a cargo compartment.
In addition, commercial passenger aircraft usually are equipped with a so-called supplemental cooling system which serves to cool, for example, food that is stored on board the aircraft in the region of the aircraft galleys either in cooled galley compartments or mobile trolleys. Moreover, the supplemental cooling system may be used to cool heat generating components such as, for example, electric or electronic components on board the aircraft. Supplemental cooling systems as described, for example, in DE 43 40 317 C2 and U.S. Pat. No. 5,513,500, in EP 1 979 233 B1 and US 2009/000329 A1, in DE 10 2006 005 035 A1 and WO 2007/088012 A1 or in DE 10 2009 011 797 A1 and US 2010/0251737 A1 comprise a plurality of cooling stations which are arranged close to the cooling energy consumers, for example in the galley regions of the aircraft, and which are provided with cooling energy from a central refrigerating device. The cooling stations are thermally connected to the central refrigerating device via a cooling circuit which is operated with either a one-phase liquid refrigerant or a two-phase refrigerant, which upon taking up heat from the cooling stations is transferred from the liquid state of aggregation to the gaseous state of aggregation and which again is transferred back from the gaseous state of aggregation to the liquid state of aggregation due to the transfer of cooling energy generated by the central refrigerating device.
The embodiments described herein are directed at the object of providing an aircraft air conditioning system which is operable with high energy efficiency and thus reduces the fuel consumption of an aircraft equipped with the aircraft air conditioning system. Further, the embodiments are directed to provide a method of operating an aircraft air conditioning system of this kind.
An aircraft air conditioning system comprises a process air source. The process air source preferably is adapted to provide process air at an elevated pressure, for example at a pressure of about 2 bar. Furthermore, the process air provided by the process air source may have a temperature of about +200° C. The aircraft air conditioning system may use an engine or an auxiliary power unit as the process air source. In particular, the aircraft air conditioning system may be provided with process air from an engine of the aircraft during flight operation of the aircraft. During ground operation of the aircraft, the aircraft air conditioning system preferably is provided with process air from an auxiliary power unit of the aircraft or an aircraft external ground support equipment.
An air conditioning unit of the aircraft air conditioning system is connected to the process air source via a process air line. A flow control valve may be provided in the process air line for controlling a mass flow of the process air through the process air line. The air conditioning unit is adapted to cool and expand process air supplied to the air conditioning unit from the process air source so as to provide conditioned air. For example, the air conditioning unit may comprise at least one heat exchanger as well as compression and expansion units. The at least one heat exchanger of the air conditioning unit may be arranged within a ram air channel or may be otherwise provided with ram air in order to discharge heat from the heat exchanger. The conditioned air exiting the air conditioning unit typically has a temperature of approximately −20° C. and a pressure of approximately 1 bar. Furthermore, as compared to the mass flow of the process air through the process air line, the mass flow of the conditioned air exiting the air conditioning unit is low.
The aircraft air conditioning system further comprises a mixing chamber which is connected to the air conditioning unit via a conditioned air line. The mixing chamber is adapted to mix conditioned air supplied to the mixing chamber from the air conditioning unit with recirculation air recirculated from an aircraft region to be air conditioned via a recirculation air line so as to provide mixed air. The aircraft region to be air conditioned which is supplied with mixed air from the mixing chamber may, for example, be a passenger cabin and/or a cargo compartment of the aircraft. A suitable conveying device such as, for example, a recirculation fan, may be disposed in the recirculation air line in order to convey recirculation air discharged from the aircraft region to be air conditioned, for example the passenger cabin of the aircraft, to the mixing chamber. Within the mixing chamber, conditioned air and recirculation air may be mixed in dependence on the ambient conditions, the operating state of the air conditioning unit and/or the cooling demand of the aircraft region to be air conditioned in such a manner that mixed air to be supplied to the aircraft region to be air conditioned is provided at a desired set temperature and with a desired set mass flow rate.
A mixed air line connects the mixing chamber to the aircraft region to be air conditioned. The mixed air line is adapted to supply mixed air provided by the mixing chamber to the aircraft region to be air conditioned. For example, the mixed air line may connect the mixing chamber to a passenger cabin and/or a cargo compartment of the aircraft. The mixed air line may be connected to the process air line via a first trim air line. Via the first trim air line, process air provided by the process air source, i.e. for example bleed air bled off from an engine or an auxiliary power unit of the aircraft or process air provided by a ground support equipment, may be supplied to the mixed air line downstream of the mixing chamber in order to adjust the temperature and/or the mass flow rate of the mixed air before finally being supplied to the aircraft region to be air conditioned.
The aircraft air conditioning system according to the invention further comprises at least one heat exchanger which is adapted to establish a thermal coupling between the aircraft air conditioning system and an external cooling energy source. Further, the heat exchanger is adapted to transfer cooling energy provided by the external cooling energy source to the aircraft air conditioning system. As compared to prior art systems, the aircraft air conditioning system according to the invention thus is not limited to the use of the air conditioning unit as the only cooling energy source. Instead, the aircraft air conditioning system, via the heat exchanger, at least in certain operational states of the aircraft air conditioning system, may be supplied with additional cooling energy provided by the external cooling energy source. For example, the external cooling energy source may be used to supply cooling energy to the aircraft air conditioning system during a peak load operational state of the aircraft air conditioning system thus allowing an appropriate adjustment of the overall layout of the aircraft air conditioning system. As a result, the aircraft air conditioning system can be designed more lightweight and operated more efficiently.
Furthermore, during flight operation of an aircraft equipped with the aircraft air conditioning system, due to the external cooling energy source providing additional cooling energy to the aircraft air conditioning system, the ram air demand of the air conditioning unit of the air conditioning system can be reduced. As a result, the aerodynamic drag caused by the ram air channel supplying ram air to the heat exchanger(s) of the air conditioning unit and hence the fuel consumption of the aircraft can be reduced. Moreover, a lower cooling energy demand allows a reduction of the mass flow rate of process air supplied from the process air source to the air conditioning unit of the air conditioning system. As a consequence, less bleed air has to be bled off the main engines of the aircraft during flight operation, thus allowing an increase of the efficiency of the engines. During ground operation of the aircraft, a lower process air demand allows the realisation of energy savings for operating the auxiliary power unit and/or an external ground support equipment.
In principle, the aircraft air conditioning system may be supplied with additional cooling energy from various types of external cooling energy sources. The aircraft air conditioning system may be provided with only one heat exchanger which is adapted to establish a thermal coupling between the aircraft air conditioning system and only one external cooling energy source. It is, however, also conceivable to provide the aircraft air conditioning system with a plurality of heat exchangers which are adapted to establish a thermal coupling between the aircraft air conditioning system and a single external cooling energy source or a plurality of (various) external cooling energy sources. Preferably, a system is used as the external cooling energy source which is already present on board the aircraft and which, at least during certain operational states of the aircraft, has excess cooling capacities which may be allocated to the aircraft air conditioning system.
In a preferred embodiment, the aircraft air conditioning system comprises at least one first heat exchanger which is adapted to establish a thermal coupling between the aircraft air conditioning system and a supplemental cooling system of the aircraft. If desired, a plurality of first heat exchangers may be provided which may be arranged at different positions in the aircraft air conditioning system so as to supply cooling energy provided by the supplemental cooling system to the aircraft air conditioning system at different positions. The supplemental cooling system may comprise at least one cooling station which is adapted to provide cooling energy to a cooling energy consumer installed on board an aircraft. The cooling energy consumer may, for example, be a galley region to be cooled, i.e. for example a galley compartment or a mobile trolley which is used for storing food to be supplied to the passengers on board the aircraft. It is, however, also conceivable that a cooling station of the supplemental cooling system serves to cool heat generating components, for example electric or electronic components installed on board the aircraft. Typically, the supplemental cooling system comprises a plurality of cooling stations which may be arranged in a forward region and/or an aft region of the aircraft cabin.
Furthermore, the supplemental cooling system may comprise a central refrigerating device which is thermally coupled to the at least one cooling station via a cooling circuit. The central refrigerating device may be installed, for example, in the region of a Belly Fairing of the aircraft, i.e. remote from the cooling stations. For example, the central refrigerating device may be designed in the form of a vapour-compression refrigeration device. The cooling circuit may allow the circulation of a one-phase liquid refrigerant or the circulation of a two-phase refrigerant there through.
Typically, the supplemental cooling system of an aircraft is designed to be able to provide a sufficient amount of cooling energy to the cooling energy consumers associated with the supplemental cooling system also during ground operation of the aircraft when the aircraft air conditioning system is not running. In other words, the supplemental cooling system is designed so as to be capable of cooling for example the galley regions of the aircraft when the aircraft is on the ground also on hot days when the temperature within the aircraft cabin may exceed 50° C. as long as the aircraft air conditioning system is not in operation. As a consequence, the supplemental cooling system, during flight operation of the aircraft, when the aircraft air conditioning system is running anyway, typically has excess cooling capacities which, via the at least one first heat exchanger, can be allocated to the aircraft air conditioning system.
The at least one first heat exchanger may be adapted to establish a thermal coupling between the aircraft air conditioning system and the cooling circuit of the supplemental cooling system. The installation position of the first heat exchanger then can be selected in a rather flexible way anywhere in the cooling circuit of the supplemental cooling system. In addition, a thermal coupling between the heat exchanger and the cooling circuit can be realized in a simple manner with the aid of suitable control valves which are adapted to control the supply of refrigerant flowing through the cooling circuit to the first heat exchanger. If desired, the cooling circuit of the supplemental cooling system may be provided with an additional bypass line such that the refrigerant flowing through the cooling circuit of the supplemental cooling system may be directed to the first heat exchanger or not as desired.
Preferably, the at least one first heat exchanger is adapted to establish a thermal coupling between the aircraft air conditioning system and the cooling circuit of the supplemental cooling system downstream of the cooling station and upstream of the central refrigerating device, i.e. in a return flow path of the cooling circuit. Refrigerant exiting the cooling station of the supplemental cooling system typically has a temperature which is still low enough to discharge heat from the aircraft air conditioning system. Moreover, since the refrigerant, after taking up heat from the aircraft air conditioning system in the first heat exchanger, is supplied to the central refrigerating device, by suitably controlling the operation of the central refrigerating device, a negative impact on the cooling performance of the cooling station can be prevented, since the refrigerant, in the central refrigerating device, may then be cooled to a desired low temperature before again being supplied to the cooling station. Finally, supplying the refrigerant to the refrigerating device of the supplemental cooling system at a higher temperature allows an increase in the operational efficiency of the refrigerating device and hence the overall supplemental cooling system.
Preferably, the at least one first heat exchanger is arranged in the mixed air line and/or the recirculation air line of the aircraft air conditioning system. Of course, one first heat exchanger may be arranged in the mixed air line and one first heat exchanger may be arranged in the recirculation air line of the aircraft air conditioning system. Alternatively or additionally thereto, the first heat exchanger may be arranged in a connecting line of the aircraft air conditioning system which connects a first aircraft region to be air conditioned, for example a passenger cabin of the aircraft, to a second aircraft region to be air conditioned, for example a cargo compartment of the aircraft, and which is adapted to supply air discharged from the first aircraft region to be air conditioned to the second region to be air conditioned. The mixed air flowing through the mixed air line, the recirculation air flowing through the recirculation air line and the air discharged from the first aircraft region to be air conditioned and flowing through the connecting line has a temperature which is higher than the temperature of the refrigerant flowing through the return flow path of the cooling circuit of the supplemental cooling system after exiting the cooling station. The refrigerant therefore may be used to efficiently discharge heat from the mixed air, the recirculation air and the air discharged from the first aircraft region to be air conditioned. Furthermore, the mass flow rate of the mixed air flowing through the mixed air line, the recirculation air flowing through the recirculation air line and the air discharged from the first aircraft region to be air conditioned and flowing through the connecting line is within a range that allows the mixed air, the recirculation air and the air discharged from the first aircraft region to be air conditioned to be cooled by heat transfer to the refrigerant circulating in the cooling circuit of the supplemental cooling system.
In a preferred embodiment, the aircraft air conditioning system further comprises a control unit which is adapted to control the operation of the aircraft air conditioning system and the supplemental cooling system in such a manner that the cooling energy provided by the refrigerating device of the supplemental cooling system is supplied exclusively to the cooling station of the supplemental cooling system until a temperature of the cooling energy consumer supplied with cooling energy by the cooling station has reached a predetermined set temperature. Further, the control unit may be adapted to control the operation of the aircraft air conditioning system and the supplemental cooling system in such a manner that at least a part of the cooling energy provided by the refrigerating device of the supplemental cooling system is allocated to the aircraft air conditioning system when the temperature of the cooling energy consumer supplied with cooling energy by the cooling station has reached the predetermined set temperature.
This control strategy ensures that the primary task of the supplemental cooling system, providing cooling energy to the cooling energy consumer associated with the supplemental cooling system, is not affected by allocating cooling energy generated by the refrigerating device of the supplemental cooling system to the aircraft air conditioning system. Instead, a cooling energy allocation takes place only when sufficient cooling of the cooling energy consumer is ensured.
Alternatively or additionally thereto, the control unit may be adapted to control the operation of the aircraft air conditioning system and the supplemental cooling system in such a manner that the cooling energy provided by the refrigerating device of the supplemental cooling system is supplied exclusively to the cooling station of the supplemental cooling system until a change in the cooling performance of the supplemental cooling system over a predetermined period of time falls below a predetermined threshold value. Furthermore, the control unit may be adapted to control the operation of the aircraft air conditioning system and the supplemental cooling system in such a manner that at least a part of the cooling energy provided by the refrigerating device of the supplemental cooling system is allocated to the aircraft air conditioning system when the change in the cooling performance of the supplemental cooling system over a predetermined period of time has fallen below the predetermined threshold value.
This control strategy ensures that cooling energy generated by the refrigerating device of the supplemental cooling system is allocated to the aircraft air conditioning system only when the performance of the supplemental cooling system has converged to a stationary stage, i.e. only little changes in the performance of the supplemental cooling system over time occur. When the cooling performance of the supplemental cooling system is more or less constant, a more or less constant amount of cooling energy can be allocated to the aircraft air conditioning system allowing the aircraft air conditioning system to be operated also at a stationary state with almost constant performance. The operational efficiency of the aircraft air conditioning system thus can be maximized.
The operation of the aircraft air conditioning system and the supplemental cooling system can be controlled according to only one of the above described cooling strategies. It is, however, also conceivable to switch between the two control strategies as desired. For example, upon start-up of the aircraft air conditioning system and the supplemental cooling system, first the “single load” control strategy can be performed wherein cooling energy generated by the refrigerating device of the supplemental cooling system is supplied exclusively to the cooling station of the supplemental cooling system until a temperature of the cooling energy consumer supplied with cooling energy by the cooling station has reached a predetermined set temperature. When both, the aircraft air conditioning system and the supplemental cooling system, have reached a more or less stationary operational state, the control strategy may be changed to the “load sharing mode”, wherein cooling energy generated by the refrigerating device of the supplemental cooling system is allocated to the aircraft air conditioning system, provided a change of the cooling performance of the supplemental cooling system over a predetermined period of time is below a predetermined threshold value. However, independent of the selected control strategy, during operation of the aircraft air conditioning system and the supplemental cooling system, the cooling energy demand of the cooling energy consumer associated with the supplemental cooling system should be prioritized over the cooling energy demand of the aircraft air conditioning system so as to ensure that the cooling energy consumer is sufficiently cooled.
The control unit further may be adapted to control the operation of the aircraft air conditioning system in dependence on the cooling energy amount allocated to the aircraft air conditioning system from the external cooling energy source. For example, the control unit may be adapted to reduce the cooling energy output of the air conditioning unit of the aircraft air conditioning system in dependence on the amount of cooling energy transferred to the aircraft air conditioning system from the external cooling energy source.
The aircraft air conditioning system may further comprise at least one second heat exchanger which is designed in the form of a skin heat exchanger, i.e. a heat exchanger which forms a part of the aircraft outer skin and hence is cooled by the ambient air flowing over the aircraft outer skin. A second heat exchanger designed in the form of a skin heat exchanger thus is adapted to establish a thermal coupling between the aircraft air conditioning system and ambient air. In this embodiment of the aircraft air conditioning system, the external cooling energy source thus is the ambient air cooling the skin heat exchanger. A second heat exchanger designed in the form of a skin heat exchanger preferably is arranged in the process air line of the aircraft air conditioning system where the air has a high temperature, a high pressure and a high mass flow rate. The aircraft air conditioning system may be equipped with only at least one second heat exchanger. It is, however, also conceivable to use the ambient air, beside the supplemental cooling system of the aircraft, as a second external cooling energy source.
Furthermore, the aircraft air conditioning system may comprise at least one third heat exchanger which is adapted to establish a thermal coupling to a refrigerating device and/or a heat pump. In this embodiment, the refrigerating device and/or the heat pump act(s) as the external cooling energy source, wherein the refrigerating device and/or the heat pump may constitute the only external cooling energy source providing additional cooling energy to the aircraft air conditioning system or may be provided in addition to the supplemental cooling system and/or the ambient air as described above. The at least one third heat exchanger may be arranged in the conditioned air line and/or a conditioned air branch line branching off from the conditioned air line and connecting the conditioned air line to the aircraft region to be air conditioned, in particular a second aircraft region to be air conditioned which is constituted by the cargo compartment of the aircraft. As already indicated above, the temperature of the conditioned air flowing through the conditioned air line and/or the conditioned air branch line has a temperature of approximately −20°. The refrigerating device and/or the heat pump therefore should be designed so as to be able to cool the conditioned air to such a low temperature. As compared to a refrigerating device, a heat pump has the advantage that it may also be operated so as to heat the conditioned air flowing through the conditioned air line and/or the conditioned air branch line, if required.
A method of operating an aircraft air conditioning system comprises the steps of supplying process air from a process air source to an air conditioning unit via a process air line, cooling and expanding process air by the air conditioning unit so as to provide conditioned air, and supplying conditioned air from the air conditioning unit to a mixing chamber via a conditioned air line. The conditioned air supplied to the mixing chamber from the air conditioning unit is mixed with recirculation air recirculated from an aircraft region to be air conditioned to the mixing chamber via a recirculation air line so as to provide mixed air. From the mixing chamber, mixed air is supplied to the aircraft region to be air conditioned via a mixed air line. A thermal coupling is established between the aircraft air conditioning system and an external cooling energy source by means of at least one heat exchanger. Cooling energy provided by the external cooling energy source is transferred to the aircraft air conditioning system via the at least one heat exchanger.
At least one first heat exchanger may establish a thermal coupling between the aircraft air conditioning system and a supplemental cooling system. The supplemental cooling system may comprise at least one cooling station being adapted to provide cooling energy to a cooling energy consumer installed on board the aircraft and a central refrigerating device being thermally coupled to the cooling station via a cooling circuit.
The at least one first heat exchanger may establish a thermal coupling between the aircraft air conditioning system and the cooling circuit of the supplemental cooling system, preferably downstream of the cooling station and upstream of the central refrigerating device.
The at least one first heat exchanger may be arranged in the mixed air line, the recirculation air line and/or a connecting line connecting a first aircraft region to be air conditioned to a second aircraft region to be air conditioned and being adapted to supply air discharged from the first aircraft region to be air conditioned to the second aircraft region to be air conditioned.
The operation of the aircraft air conditioning system and the supplemental cooling system may be controlled in such a manner that the cooling energy provided by the refrigerating device of the supplemental cooling system is supplied exclusively to the cooling station of the supplemental cooling system until a temperature of the cooling energy consumer supplied with cooling energy by the cooling station has reached a predetermined set temperature, and that at least a part of the cooling energy provided by the refrigerating device of the supplemental cooling system is allocated to the aircraft air conditioning system when the temperature of the cooling energy consumer supplied with cooling energy by the cooling station has reached the predetermined set temperature.
Alternatively or additionally thereto, the operation of the aircraft air conditioning system and the supplemental cooling system may be controlled in such a manner that the cooling energy provided by the refrigerating device of the supplemental cooling system is supplied exclusively to the cooling station of the supplemental cooling system until a change in the performance of the supplemental cooling system over a predetermined period of time falls below a predetermined threshold value, and that at least a part of the cooling energy provided by the refrigerating device of the supplemental cooling system is allocated to the aircraft air conditioning system when the change in the cooling performance of the supplemental cooling system over a predetermined period of time has fallen below the predetermined threshold value.
Preferably, the operation of the aircraft air conditioning system is controlled in dependence on the cooling energy amount supplied to the aircraft air conditioning system from the external cooling energy source.
At least one second heat exchanger may be designed in the form of a skin heat exchanger which establishes a thermal coupling to ambient air. The at least one second heat exchanger preferably is arranged in the process air line.
Preferably, at least one third heat exchanger is provided which establishes a thermal coupling to a refrigerating device and/or a heat pump. The at least one third heat exchanger preferably is arranged in the conditioned air line and/or a conditioned air branch line branching off from the conditioned air line and connecting the conditioned air line to the aircraft region to be air conditioned.
A preferred embodiment will now is described in greater detail with reference to the appended schematic drawings, wherein